The present invention relates to plating integrated circuit packages, and more particularly to electroplating traces on an integrated circuit (IC) substrate using a removable plating bus.
In the last few decades, the electronics industry has literally transformed the world. Electronic devices are used by, or affect the daily lives of, a large segment of the world's population. For example, telephones, television, radios, Personal Computers (PCs), laptop PCs, palmtop PCs, PCs with built-in portable phones, cellular phones, wireless phones, pagers, modems, and video camcorders, are just a few of the electronic products that have been developed in recent years and which have been made smaller and more compact, while providing more and/or enhanced functions than ever before. The integrated circuit (IC) chip, and the more efficient packaging of the IC chip, have played a key role in the success of these devices.
Integrated circuit substrates are used to connect the IC chip to the electronic package. One popular group of packages is the Grid Array package, which consists of an IC substrate for mounting and interconnecting the IC chip to the electronic package. On the top surface of the IC substrate, an IC chip is mounted and electrically connected to traces that are formed on the top surface of the IC substrate. On the bottom surface of the IC substrate, pins or solder balls are mounted in an array pattern for connection of the electronic package.
The IC chip is typically attached in the center of the IC substrate. Wires are used to connect a plurality of contact pads on the IC chip to lands on the IC substrate. The lands are typically the ends of the traces near the IC chip. Alternatively, the IC chip can be connected directly to the lands by solder bumps (this is known as a flip-chip connection). The lands are in turn connected to the array of pins or balls on the opposite surface on the IC substrate by metal traces and vias through the IC substrate. The metal traces are typically made of copper.
For wire bonding or bump attachment of the IC chip, the lands should be plated with gold, nickel or silver. The preferred method of plating is electroplating. Electroplating is easily controlled, efficient and cost effective. During the electroplating process, conductive material is deposited in preselected areas. Electroplating requires that an electrically conductive connection be made to all the lands and metal traces that are to be plated. To prevent the conductive material from piling on undesired areas, the traces and IC substrate are covered so that the plating will not stick. This can be done with a plating resist material that is applied to the IC substrate. In certain areas where the plating material is desired, the resist material is removed. The plating solution bath and the exposed area on the conductive traces or lands function as a pair of electrodes. An electric current flows from an outside source through traces to deposit desired conductive material on the selected areas of the IC substrate.
To electrically connect the lands and traces together, the traces are typically extended to the outside edge of the IC substrate where they are connected to a plating bus, which is used only for plating. After the electroplating, the plating bus is removed before the electronic package is finished. For low density packages, it is relatively easy to incorporate the bus in the design of the IC substrate. For very dense designs, the metal traces on the IC substrate are spaced very close together. In some cases, it is not possible to route all the metal traces to the edge of the package for connection to the bus.
In the case where an independent conductive trace cannot be extended to the periphery of the circuit board to connect to the bus, the trace is temporally connected to another close trace by an additional conductive pattern so that an electrical current flows from a peripheral conductive pattern through the desired conductive segment. Such an additional conductive pattern, which is unnecessary in the final IC substrate configuration, is called a "plating lead pattern". The plating lead patterns are only necessary as an intermediate step in the plating step of the IC substrate. Once the plating is done, the traces must again be electrically isolated. Thus, the electrical interconnection between the independent conductive traces must be removed before the IC substrate is finished. The plating bus and plating lead patterns add complexity and cost to the manufacture of the IC substrate.
In view of the above, it is evident that what is needed is a low-cost method of electroplating select areas of a high density IC substrate without the need for a permanent plating bus or the addition of plating lead patterns.